Why flat roofs fail in Las Vegas, and what actually fixes them

Flat and low-slope roofs are everywhere in this valley: mid-century homes, additions, garages, carports, patio covers and nearly every small commercial building. They fail differently from pitched roofs, and they are frequently misdiagnosed by people who mostly work on pitched ones.

The fundamental difference is simple. A pitched roof sheds water by geometry, so the covering can be imperfect and still work. A low-slope roof holds water, so it depends entirely on a continuous membrane and functioning drainage. There is much less margin for error.

Reading the symptoms

Each of these means something specific, and the distinction decides whether you need a repair, a coating or a replacement:

• Ponding water remaining more than a day or two: a drainage fault, not usually a membrane fault
• Blistering: moisture or vapour trapped beneath the membrane expanding in the heat and lifting it
• Open or shrunken seams and laps: the most common actual entry point on rolled and modified bitumen roofs
• Alligatoring or fine crazing: ultraviolet degradation of the surface, often a recoat signal rather than a failure
• Soft or spongy areas underfoot: saturated insulation beneath, which is a serious finding
• Punctures and gouges: usually foot traffic or dropped tools from equipment servicing
• Separated or lifted flashing at parapet walls: a perennial low-slope weak point
• Exposed foam on a sprayed polyurethane roof: the protective coating has worn through and the foam is now degrading

Ponding is the one to take seriously

A low-slope roof is designed to drain, not to hold water. Standing water adds weight, accelerates membrane degradation and eventually finds any weakness in a seam. In this climate it also evaporates and leaves concentrated debris and mineral deposits behind, which hold moisture longer next time.

Persistent ponding almost always means a drainage problem: a blocked outlet, a sagging deck, or insufficient slope built in originally. Coating over a ponding area without addressing the drainage treats the symptom and leaves the cause in place.

Foam roofs and their coating

Sprayed polyurethane foam is widely used on valley flat roofs and performs well, with one important caveat: the foam itself is not ultraviolet stable. It depends on a protective coating, and that coating is a maintenance item with a finite life, not a permanent finish.

A foam roof whose coating has worn through starts degrading underneath, and that is a substantially more expensive problem than recoating on schedule. If you have a foam roof and cannot remember when it was last coated, that is worth finding out.

When coating is the right answer, and when it is not

Coating is genuinely good on a sound but ageing membrane, on a foam roof due for its scheduled recoat, and where reflectivity is wanted to cut the solar load into the building.

It is the wrong answer over saturated insulation, where it seals the water in. It is the wrong answer on a ponding area where the drainage has not been fixed. And it is the wrong answer on a membrane that has already failed structurally, where it buys a season at best.

A contractor who inspects the roof, checks for trapped moisture and tells you it needs replacing rather than coating is giving you the more valuable answer, even though it is the more expensive one.

Preparation decides whether a coating lasts

Coating failures are overwhelmingly preparation failures. The surface has to be genuinely clean, dry and sound, defects have to be repaired first, and the right primer has to be used for that substrate.

A coating quote should be explicit about cleaning and preparation, which defects are being repaired beforehand, the product and number of coats, the dry film thickness being achieved, and the warranty on the system. A coating quote well below the others usually differs in exactly those places.

Rooftop equipment is the other main culprit

On any flat roof carrying air conditioning, refrigeration or exhaust equipment, a large share of the damage comes from the people servicing it rather than from weather. Technicians who are not roofers walk membranes, drop tools and leave condensate discharging in the wrong place.

If your building has rooftop equipment, it is worth inspecting the roof after service visits rather than only after storms.